Short communication
An Efficient Microwave-Induced Solvent-Free Organic Bromination Using Tetrabutylammonium
Tribromide
Alimenla Bernard, Anil Kumar, Latonglila Jamir, Dipak Sinha and Upasana Bora Sinha*
Department of Chemistry, Nagaland University, Lumami Campus, Nagaland-798601, India
* Corresponding author: E-mail: upasanaborasinha@gmail.com
Received: 29-08-2008
Abstract
Microwave-induced solvent-free brominations of organic substrates have been carried out with tetrabutylammonioum tribromide, (C4H9)4N+(Br3-). Reactions are facile, affording products in high yields within very short reaction times.
Keywords: Microwave (MW); bromination; tetrabutylammonium tribromide (TBATB); environmentally benign reaction
1. Introduction
In recent times, due to growing environmental awareness1-3 microwave-induced chemical synthesis has become a useful alternative to conventional synthetic proce-dures.4-7 In this context a few reports are also available on microwave-induced organic brominations with reagents such as bromine and NBS.8-11 However, these days, organic bromination reactions are often carried out using orga-nicammonium tribromides (OATBs).12-16 Among the different OATBs developed so far, tetrabutylammonium tri-bromide (TBATB), (C4H9)4N+(Br3-), is the most versatile reagent.17-29 TBATB is regarded as a 'greener' substitute of bromine because it is stable, crystalline, easy to handle and has worked well as a brominating agent, allowing bromination of a wide array of organic substrates.30 In an effort to further explore the scope of TBATB as bromina-ting reagent, we recently performed some reactions under microwave conditions using a few test substrates. Encouraged by the initial results,31 a detailed study has now been done and reported in this communication.
2. Experimental
The reagent, tetrabutylammonium tribromide (TBATB), (C4H9)4N+(Br3-), used for these bromination
reactions was synthesized following one of our already reported preparation procedures.23
2. 1. General Procedure for Bromination Reactions Under Microwave Conditions
The reactions were carried out in a monomode reactor (Synthewave 402® Prolabo). In a typical reaction, 1 mmol of organic substrate was mixed thoroughly with 1 mmol of TBATB, with light grinding in order to ensure complete homogeneity. The mixture was then inserted into the microwave chamber and the reaction was carried out maintaining the power at 40% and reaction temperature at 90 °C. The progress of the reaction was monitored by TLC using ethylacetate-hexane solvent system (volume ratio varied for different substrates). Once the reaction was completed, the reaction mixture was diluted with ethyl acetate (30 mL) and filtered through a short column of silica gel in order to remove the spent reagent. The crude product thus obtained was subjected to column chromatography over a pad of silica gel using ethyl acetate-hexane solvent system as eluent. The products were identified by comparison of their melting points, IR absorption and NMR spectra with the authentic samples.32
3. Results and Discussion
In order to study the versatility of TBATB as bromi-nating agent under microwave conditions, representative examples of different types of organic substrates were taken (see Table 1). It was observed that bromination reac-
tions under microwave conditions were very facile and the products were obtained in moderate to high yields. Similar experiments performed after homogenization of the reaction mixtures by grinding, followed by conventional heating at 90 °C showed no reaction within the same reaction time as needed for microwave conditions.
Table 1: Microwave accelerated bromination of organic substrates using TBATB under neat reaction conditions.a
Entry Substrate
Product
Time (s) Yieldb (%)
120
62
93
92


,NH2
Br"
50
83
OoN

70
85
,nh,
CO2H
Br
,NH2 ^C02H
0
55

V
Br'
93
85
^OH

9
76
336
60
265
72
10
93
70
1
8
9
Entry Substrate
Product	Time (s) Yieldb (%)
11
153	81
12
60	55
13
76	45
14
196	40
15
110	35
16
60	95
17
180	90
18
120	65
19
60	70
20
213	56
21
60	70
22
83	87
Entry Substrate
Product
Time (s) Yieldb (%)
23
90
68
24
160
62
25
105
72
aReaction conditions: microwave power, 40%; temperature, 90 °C; solvent-free conditions. ^Isolated yields.
As we can see from Table 1, when reactions were carried out on unsubstituted aromatic compounds like naphthalene and anthracene (entries 1 and 2), only mono-brominated products were obtained. With aromatic amino compounds such as aniline, 4-nitroaniline, 2-aminoben-zoic acid and acetanilide (entries 3 to 6), regioselectivity was maintained and monobrominated products were formed in moderate to high yields. Microwave bromination reactions were also performed on phenol and some of its derivatives (entries 7 to 11). In case of cresols, only monobromo products were obtained regioselectively, which is probably because of the higher o,,p-directing power of the OH substituent. However, it has been reported earlier that mixtures of products are obtained when liquid bromine is used as reagent.12 Fused ring phenolic compounds (a- and ß-naphthols, entries 10 and 11) afforded the expected bro-mo-products in high yields. Bromination was also possible on deactivated benzene systems such as chloro and nitrobenzene (entries 12 and 13). Moderate yields of products were also obtained with benzoic acid (entry 14) and 4-nitrobenzoic acid (entry 15). For multiple bonded systems (entries 16-18) the corresponding trans-products were obtained in high yields. a-Bromination of ketones was possible by this method as indicated by reactions of cyclohexanone and 4-nitroacetophenone (entries 19 and 20), and a,ß-unsaturated systems of different types (entries 21-24) gave the corresponding dibromo products in good yields. Finally, bromination of heterocyclic compounds, such as imidzole, is also possible (entry 25).
4. Conclusions
Microwave-induced brominations with TBATB were successfully carried out on different types of organic substrates under solvent-free conditions. This method could be useful alternative to the existing methods of bro-
mination, because the reactions are facile, with extremely short reaction times and simple purification process, and products are obtained in moderate to high yields.
5. Acknowledgements
The investigation has enjoyed financial support from DST, New Delhi (No.SR/S1/IC-52/2003).
6. References
1.	J. H. Clark, Green Chem. 1999, 1, 1-8.
2.	P. T. Anastas, J. C. Warner: Green Chemistry: Theory and Practice. Oxford University Press, Oxford, U.K., 2000, pp. 20-33.
3.	See Thematic Issue on Green Chemistry, Chem. Rev. 2007, 107, 2167-2708.
4.	V. Polshettiwar, R. S. Varma, Acc. Chem. Res. 2008, 41, 629-639.
5.	B. P. Mason, K. E. Price, J. L. Steinbacher, A. R. Bogdan, D. T. McQuade, Chem. Rev. 2007, 107, 2300-2318.
6.	P. Lidstrom, J. Tierney, B. Wathey, J. Westman, Tetrahedron 2001, 57, 9225-9283.
7.	M. Nüchter, B. Ondruschka, W. Bonrath, A. Gum, Green Chem. 2004, 6, 128-141.
8.	S. Goswami, S. Dey, S. Jana, A. K. Adak, Chemistry Lett.
2004,	33, 916-917.
9.	C. Kuang, Q. Yang, H. Senboku, M. Tokuda, Tetrahedron
2005,	61, 4043-4052.
10.	S. Paul, V. Gupta, R. Gupta, A. Loupy, Tetrahedron Lett, 2003, 44, 439-442.
11.	V. L. Budarin, J. H. Clark, S. J. Tavener, K. Wilson, J. Chem. Soc. Chem. Commun. 2004, 2736 -2737.
12.	V. Kavala, S. Naik, B. K. Patel, J. Org. Chem. 2005, 70, 4267-4271.
13.	A. D. Jordan, C. Luo, A. B. Reitz, J. Org. Chem. 2003, 68, 8693-8696.
14.	C. Chiappe, E. Leandri, D. Pieraccini, J. Chem. Soc. Chem. Commun. 2004, 2536 -2537.
15.	J. C. Lee, J. Y. Park, S. Y. Yoon, Y. H. Bae, S. J. Lee, Tetrahedron Lett. 2004, 45, 191 -193.
16.	S. Singhal, S. L. Jain, B. Sain, J. Mol. Catal. A; Chem. 2006, 258, 198-202.
17.	M. K. Chaudhuri, A. T. Khan, B. K. Patel, D. Dey, W. Khar-mawphlang, T. R. Lakshmiprabha, G. C. Mandal, Tetrahedron Lett. 1998, 39, 8163-8166.
18.	A. D. Jordan, C. Luo, A. B. Reitz, J. Org. Chem. 2003, 68, 8693-8696.
19.	J. Salazar, R. Dorta, Synlett 2004, 1318-1320.
20.	K. Tanaka, R. Shiraishi, F. Toda, J. Chem. Soc., Perkin Trans. 1 1999, 3069-3070.
21.	E. Mondal, G. Bose, A. T. Khan, Synlett 2001, 785-786.
22.	G. Bose, Y. Li, P. M. Bujarbarua, D. Kalita, A. T. Khan, Chem. Lett. 2001, 30, 290-291.
23.	U. Bora, M. K. Chaudhuri, D. Dey, S. S. Dhar, Pure Appl.Chem. 2001, 73, 93 -102.
24.	G. Bose, E. Mondal, A. T. Khan, M. J. Bordoloi, Tetrahedron Lett. 2001, 42, 8907-8909.
25.	S. Naik, V. Kavala, R. Gopinath, B. K. Patel, Arkivoc 2006, (i), 119-127.
26.	E. Mondal, P. R. Sahu, G. Bose, A. T. Khan, Tetrahedron Lett. 2002, 43, 2843-2846.
27.	J. Gosain, P. K. Sharma, Indian Acad. Sci. (Chem. Sci.) 2003, 115, 135-145.
28.	V. Kavala, B. K. Patel, Eur. J. Org. Chem. 2005, 441-443.
29.	S. Naik, R. Gopinath, B. K. Patel, Tetrahedron Lett. 2001, 42, 7679-7681.
30.	V. B. C. Figueira, Synlett 2006, 16, 2681-2682.
31.	B. Alimenla, A. Kumar, L. Jamir, D. Sinha, U. B. Sinha, Rad. Eff. Def. Sol. 2006, 12, 687-693.
32.	J. Buckingham, F. McDonald (Eds.): Dictionary of Organic Compounds, 6'h ed., Chapman & Hall, London, 1996.
Povzetek
Izvedli smo z mikrovalovi iducirane reakcije bromiranja organskih spojin s tetrabutilamonijevim tribromidom, (C4H9)4N+(Br3-), brez uporabe topila. Reakcije so hitre in produkti nastanejo v zelo kratkih reakcijskih časih z visokimi izkoristki.